Crossover
networks -
Nearly all speaker systems (except the “one-way” speakers) use
some type of frequency dividing network. Its purpose is to divide the audio
spectrum into frequency ranges that complement each driver. If one driver
is more sensitive than another, the frequency dividing network may also “pad”
the more sensitive driver to smooth the speaker’s response. This network
or crossover, as it is usually called, may be fairly complex made up of resistors,
inductors, and capacitors (these are known as R-L-C networks where L stands
for conductance). Some types of high frequency tweeters, such as the piezoelectric
types, naturally reject low-frequency energy, but the lack of a crossover
can be considered a shortcut that likely to produce uneven response and may
lead to driver failure.

In conventional speaker
systems, the crossover is located in the enclosure and this type of crossover
is known as the high-level crossover because it electrically joins the output
of the power amplifier (high-level audio) to the drivers. The
high-level crossover does not require any power supply, so it is known as
a passive crossover.
Even the best high-level crossover "use up" some audio power, typically
1dB. Inferior crossovers
can have insertion losses of several dB and a crossover with 3dB of loss uses
up half the power of the amplifier, before that power reaches the drivers.

The crossover network,
named for the fact that it crosses over frequencies to the proper driver,
has connections to the binding posts on the rear of the speaker, so that the
signal passes through the crossover network, and then to the speaker drivers.
The property of sending low frequencies to the woofer is called "low
pass", and the property of sending high frequencies to the tweeter is
called "high pass".
If you ever have a subwoofer, you may see these terms used. For
example, in setting the low pass frequency that a subwoofer crosses over at,
sending all signals below this frequency to the subwoofer amplifier, and the
high pass frequency, above which, signals are sent back to the main speakers.

Crossovers
slopes & frequencies
- Crossover “point” of a speaker system is a center frequency
in a range of frequencies where, as the frequency rises, the input to the
lower frequency driver is gradually attenuated and the input to the higher
frequency driver is gradually increased. "Crossover region" is the
range of frequencies where both drivers are reproducing the same frequency.
“Crossover point”, “crossover frequency” or “transition
point” is the frequency where the output from two drivers is approximately
equal. “Crossover point” may also be defined as the frequency
where each driver is being fed 3dB less power than its full out-of-the-crossover-region
power. Crossover frequencies are chosen to complement the drivers and to roughly
split the frequency spectrum into equal energy portions.

In a 3-way system, the
low-to-mid and mid-to-high frequency transitions usually occur somewhere around
800Hz and 5kHz. In
a 2-way system the low-to-high frequency transition will usually occur between
1kHz and 2kHz.
These crossover frequencies may vary considerably, depending on the system;
the crossover frequency should not be considered an index of quality. Crossover
“slope” is the rate at which the transition from one driver to
the other takes place, and is usually expressed in dB per octave. Most
common slope rates are 6dB/octave, 12dB/octave, 18dB octave, and 24dB octave.
The steeper the
slope (the larger the number), the more narrow the crossover region. Steep
crossover slopes can be beneficial because they protect the drivers better,
but the chances of phase distortion increase. Like
so many other aspects of speaker design, crossover design involves trade-offs
and in most cases, 12dB per octave is the best compromise.

Passive
vs active crossover -
Passive crossover
does not require electricity and does not use active circuitry to accomplish
its task. Found
in almost all speakers, they divide up the incoming signal and serve it to
the various speaker drivers that make up the speaker. Passive
crossovers are not adjustable beyond their factory-created settings making
them somewhat less versatile than active crossovers using electronic circuitry
to accomplish the crossover tasks. Passive
crossovers use resistors, capacitors and inductors. Active
crossover uses electronics supplied with a power source and acting on the
sound to shift sound reproduction tasks from one speaker driver to another.
Active crossovers
are the most flexible types and compared to passive crossovers, which are
fixed and use specified filters and capacitors. Active crossovers are adjustable
and use electronic circuits to split up the frequency spectrum. These
electronic circuits are often “cleaner” sounding than their passive
counterparts. Most
if not all powered subwoofers include active crossovers to split up the signal
which they will reproduce from that which should be passed on to the other
speakers.

Crossover
quality-
Active crossover
network is not better in quality than a passive unit, or vice-versa, since
these are variations of the same circuit, and are made to serve different
purposes. Quality of any crossover, passive or active, depends on its components,
its design, and its construction and you’ll probably have to find out
about a given speaker’s crossover by asking the salesperson or reading
the manufacturer’s literature. There are several criteria to use in
evaluating a crossover; self-healing capacitors and low-loss, low-distortion
inductors are desirable and resistors should have adequate power rating. To
assure similar response from each speaker in a stereo pair, crossovers should
be carefully matched as poorly matched crossovers will cause the stereo image
to jump or to be unsteady. Better crossovers will meet all these criteria,
and will introduce as little loss as possible, about 1dB (a 3dB loss would
be a loss of 50% of the amplifier’s power). Inferior
crossovers may introduce great power losses, may be more prone to “burn
out”, and may not provide the smoothness of response that might otherwise
be possible from drivers and enclosure.

Speaker
drivers
- Manufacturers often mount several different “drivers” in each
enclosure. “Driver” is a term used to describe any component of
the speaker that produces sound. In an attempt to maintain even dispersion,
lower distortion and simpler driver construction, most speaker systems divide
the frequency spectrum into two or three segments for reproduction by separate
drivers which are optimized for each segment. These are known as “2-way”
or “3-way” speaker systems. The frequency divisions are accomplished
with crossover networks (frequency dividing networks) and are chosen so that
the drivers cover approximately equal portions of the audio spectrum, by octaves.
An octave is the interval between a given tone and its repetition eight tones
above or below on the musical scale; a note which is an octave higher than
another note is twice the frequency of the first note.

An enclosure
with three or four drivers is not necessarily a 3 or 4-way speaker, since
many manufacturers use several drivers to cover the same portion of the audio
spectrum. Some
speakers even have as many as eight or more identical drivers in one cabinet,
each driver covering the full audio spectrum. A driver must move air at low
frequencies to produce the same sound level as it would when producing mid
or high frequencies. Thus, the bass driver is normally larger than the other
drivers; the mid and high-frequency drivers are not required to move as much
air, so they can be made smaller.

Small drivers actually
offer an advantage with respect to dispersion pattern and crossover networks
assign higher frequency sounds to smaller drivers so that wide dispersion
can be maintained. Physical laws show that dispersion narrows as the wavelength
of a sound approaches the diameter of the driver. Wavelength is the distance
that the beginning of a sound wave travels by the time the next cycle vibration
begins; the higher the frequency, the shorter the wavelength. This means that
with a given size driver, as the frequency goes up, the dispersion narrows
and by “crossing over” to a smaller driver, the wavelength becomes
large with respect to the driver, so dispersion is wider than it would have
been with the larger driver. Manufacturers sometimes use horn-type mid or
high-frequency drivers to control dispersion to improve the efficiency. The
horn is a funnel-like structure that flares out in front of the moving driver
element. In larger
speaker systems, horn-loaded drivers (cone type) may also reproduce the low
frequency. While
different types of drivers have characteristics separating them from each
other, compression drivers with horns, cone drivers (with or without horns),
and ribbon drivers all exhibit frequency response and dispersion which are
related to driver size.

Types of
speaker drivers -Speaker drivers can be
categorized in two ways, with regard to the frequency range they are meant
to reproduce, and with regard to their physical design. Speaker drivers meant
to reproduce bass frequencies are known as "woofers". Speaker drivers
meant to reproduce the middle frequencies are known as "midrange drivers".
Speaker drivers meant to reproduce the treble frequencies are known as "tweeters".
2-way speaker systems have at least one woofer and one tweeter. 3-way speaker
systems have at least one woofer, midrange and tweeter. 4-way speaker systems
may divide the mid-range into two segments for coverage by different drivers. Some speaker systems
utilize additional drivers called "subwoofers" for the extreme bass
region and "super tweeters" for extreme treble region.

Woofers- The
low-frequency driver is also commonly called a “woofer” and it
covers the frequency range of “bass” tones - low vocals and instruments.
Lower frequency
limit of the woofer is partly determined by the cabinet configuration, and
generally falls in the range between 30Hz and 100Hz. The
woofer’s high frequency limit usually falls somewhere between 600Hz
and 800Hz in a
3-way system, or between 1,000Hz and 2,000Hz in a 2- way system. Actual
limit for a given woofer is determined by the diameter and mass of the cone,
and by the slope and transition of the crossover network.

Midrange
drivers -Midrange frequencies are
important to most vocals and instruments, and the uppermost frequency response
of the mid-range driver begins to reproduce some of the “sibilant”
range (breath sounds in vocals, etc). “Midrange”
may be defined roughly by 500Hz (lower limit) and 5,000Hz (upper limit) and
the midrange driver covers most of the midrange frequencies, from the upper
limits of the woofer’s response, to the lower limits of the tweeter.

Tweeters- In
a 3-way speaker, the tweeter covers those frequencies above the limit of the
midrange driver, and up to 20,000Hz or higher.
In 2-way speakers, where there is no midrange driver, the tweeter also covers
the upper midrange (while the woofer cover the lower midrange). Upper
harmonics of instruments, cymbal sounds and sibilants are reproduced by the
tweeter.

Full-range
drivers -Full-range single driver
speakers attempt to cover the entire audible frequency spectrum using only
one driver unit, removing the need for an electronic crossover network, which
is well known for being hard to design without introducing colorations to
the sound. In practice,
how well a single driver speaker delivers depends both on the driver and the
speaker cabinet design. Several
types of “full-range” or “wide-range” drivers are available
and the simplest being the “coaxial” driver which consists of
a woofer with a smaller “whizzer cone” in the center. The
“whizzer” cone is fixed to the larger cone and driven by the same
voice coil, but allows the overall response to reach higher frequencies with
wider dispersion. Some
full-range designs share one magnetic assembly but use separate tweeter and
woofer sections with their own voice coils. One
such design employs a compression driver mounted behind the woofer cone, with
a throat (emerging from the center of the magnet) terminated by a small horn.

Electrostatic
drivers - The
"electrostatic" drivers cover mid or high frequencies, and sometimes
both while a few designs even attempt to reproduce bass frequencies, though
results are questionable. The electrostatic driver is constructed of two preforated plates which are
charged with high voltage at opposite polarities and a diaphragm between the plates is driven by a conventional power amplifier and
audio signals (voltages) applied to the diaphragm cause it be alternately
attracted to one plate and then the other, moving air to create sound. Excursion of an electrostatic diaphragm is limited, and therefore its low
frequency output is limited. The
electrostatic drivers also require a polarizing voltage; some units must be
plugged into the wall, others require a special amplifier with built-in power
supply, while others rectify a portion of the audio energy fed to the driver
to polarize the plates. Unless
they are very large in size, electrostatic drivers have limited sound output
but they have many advantages in that they may image better and create a more
open and expanded soundstage than conventional cone speakers.

Courtesy of HowStuffWorks

Compression
drivers -
Very high efficiencies
can be obtained with compression driver. Compression
drivers operate like cone drivers, except that the diaphragm is usually metal,
and it must be loaded by a lens or a mid or high frequency horn. Properly
designed horns and lenses have controlled, if not wide, dispersion. Major
disadvantages of compresion driver systems are their size, cost, and sometimes
limited dispersion.